Durig James R, Zheng Chao
Department of Chemistry, University of Missouri, Kansas City, MO 64110, USA.
Spectrochim Acta A Mol Biomol Spectrosc. 2007 Nov;68(3):783-95. doi: 10.1016/j.saa.2007.01.001. Epub 2007 Jan 9.
Variable temperature (-105 to -150 degrees C) studies of the infrared spectra (3500-400 cm(-1)) of ethylisothiocyanate, CH(3)CH(2)NCS, dissolved in liquid krypton have been recorded. Additionally the infrared spectra of the gas and solid have been re-investigated. These spectroscopic data indicate a single conformer in all physical states with a large number of molecules in the gas phase at ambient temperature in excited states of the CN torsional mode which has a very low barrier to conformational interchange. To aid in the analyses of the vibrational and rotational spectra, ab initio calculations have been carried out by the perturbation method to the second order (MP2) with valence and core electron correlation using a variety of basis sets up to 6-311+G(2df,2pd). With the smaller basis sets up to 6-311+G(d,p) and cc-PVDZ, the cis conformer is indicated as a transition state with all larger basis sets the cis conformer is the only stable form. The predicted energy difference from these calculations between the cis form and the higher energy trans conformer is about 125 cm(-1) which represents essentially the barrier to internal rotation of the NCS group (rotation around NC axis). Density functional theory calculation by the B3LYP method with the same basis sets predicts this barrier to be about 25 cm(-1). By utilizing the previously reported microwave rotational constants with the structural parameters predicted by the ab initio MP2(full)/6-311+G(d,p) calculations, adjusted r(0) structural parameters have been obtained for the cis form. The determined heavy atom parameters are: r(NC)=1.196(5), r(CS)=1.579(5), r(CN)=1.439(5), r(CC)=1.519(5)A for the distances and angles of angleCCN=112.1(5), angleCNC=146.2(5), angleNCS=174.0(5) degrees . The centrifugal distortion constants, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and compared to experimental quantities when available. These results are compared to the corresponding quantities of some similar molecules.
记录了溶解在液态氪中的异硫氰酸乙酯（CH₃CH₂NCS）在可变温度（-105至-150摄氏度）下的红外光谱（3500 - 400厘米⁻¹）。此外，还重新研究了该物质气体和固体的红外光谱。这些光谱数据表明，在所有物理状态下均存在单一构象异构体，在室温下的气相中有大量分子处于CN扭转模式的激发态，该模式的构象互换势垒非常低。为了辅助分析振动和转动光谱，使用高达6 - 311 + G(2df,2pd)的各种基组，通过二阶微扰方法（MP2）对价电子和内层电子关联进行了从头算。对于较小的基组（高达6 - 311 + G(d,p)和cc - PVDZ），顺式构象异构体显示为过渡态，而对于所有较大的基组，顺式构象异构体是唯一的稳定形式。这些计算预测的顺式构象异构体与能量较高的反式构象异构体之间的能量差约为125厘米⁻¹，这基本上代表了NCS基团内旋转（绕NC轴旋转）的势垒。使用相同基组通过B3LYP方法进行的密度泛函理论计算预测该势垒约为25厘米⁻¹。通过利用先前报道的微波转动常数以及从头算MP2(full)/6 - 311 + G(d,p)计算预测的结构参数，获得了顺式构象异构体的调整后r(0)结构参数。确定的重原子参数为：距离r(NC)=1.196(5)，r(CS)=1.579(5)，r(CN)=1.439(5)，r(CC)=1.519(5)埃，角度angleCCN = 112.1(5)，angleCNC = 146.2(5)，angleNCS = 174.0(5)度。已从从头算计算中预测了离心畸变常数、偶极矩、构象稳定性、振动频率、红外强度和拉曼活性，并在有可用实验量时与实验量进行了比较。将这些结果与一些类似分子的相应量进行了比较。
